1. Field of the Invention
The present invention relates to an image forming apparatus such as a printer and a copying machine, employing a method to transfer a toner image formed on an image holder onto an intermediate transfer body and transfer and fix the toner image transferred on the intermediate transfer body onto a print sheet.
2. Description of the Related Art
Recently, a so-called digital electrophotographic image forming apparatus, which forms an electrostatic latent image on a photoreceptor by emitting a light beam such as a laser beam, modulated based on image information representing characters, images and the like, on the photoreceptor, has been widely used as a printer and a copying machine.
In this digital image forming apparatus, upon formation of halftone image, as well as black-and-white binary image, halftone is represented by a known method which forms an electrostatic latent image where the gradation is represented by a so-called dot structure or line structure, by ON/OFF modulation of light beam at a high speed. The dot structure and the line structure are referred to as a halftone screen where the gradation of image, i.e., gradation representation is made by the area size of dot or line. This method uses a relatively simple algorithm, and further, it can be realized at a low cost. Accordingly, the method is widely adopted in digital electrophotographic printers and copying machines.
On the other hand, in an image forming technique to develop an electrostatic latent image on a photoreceptor with dry toner into a toner image, then electrostatically transfer the toner image onto a print sheet, and fix the transferred image, the problem is that powder toner scatters in a transfer portion and the reproducibility of a halftone representation screen (halftone screen) structure (dot structure or line structure) is degraded. This may produce an image with conspicuous graininess.
To solve this problem, Japanese Published Examined Patent Application No. Sho 46-41679 discloses adhesion-transferring a toner image formed on a photoreceptor to an intermediate transfer body and heat-melt transferring toner from the intermediate transfer body to a print sheet. This method performs toner image transfer to the print sheet in a non-electrostatic manner, which suppresses degradation of image quality at the above-described transfer process.
Further, Japanese Published Unexamined Patent Application No. Hei 2-108072 discloses a transfer fixing method of forming a multi-color multiple toner image on an intermediate transfer body by transferring a plurality of toner images in different colors, while electrostatically overlapping the toner images, onto the intermediate transfer body, further, melting the multi-color multiple toner image on the intermediate transfer body, and transferring the melted multiple toner image onto a print medium, thus obtaining a color copy. This method also transfers the multiple toner image onto the print medium in a non-electrostatic manner, therefore, the degradation of image quality as described above can be mitigated.
As an image forming apparatus using the above transfer fixing method, U.S. Pat. No. 2,990,278, Japanese Published Unexamined Patent Application No. Hei 5-19642 and Japanese Published Unexamined Patent Application No. Hei 5-249798 disclose a technique to completely perform transfer of toner image from an intermediate transfer body to a print medium, by bringing the intermediate transfer body and the print medium into tight contact and heating them, then pressing them, thereafter, cooling the intermediate transfer body and the print medium until the cohesive force between the toner particles becomes greater than the adhesive force between the toner and the intermediate transfer body, and removing the print medium from the intermediate transfer body. This technique obtains a high quality image with a high toner transfer efficiency, a good color balance, high glossiness, and excellent toner transparency. Accordingly, to effectively utilize these advantages, studies have been made especially regarding the structure of the top layer of the intermediate transfer body. Silicone rubber, having an excellent toner transfer characteristic to a print sheet, is generally used as coating material.
On the surface of the photoreceptor, a number of independent toner called fogging toner exist as well as a toner image to form an image. The silicone rubber transfers almost all the toner including the fogging toner on the photoreceptor, due to its elasticity and viscosity, onto the surface of the intermediate transfer body. If the toner image on the photoreceptor surface is directly transferred onto the print sheet without the intermediate transfer body, as in the conventional art, the transfer is selectively performed by utilizing an electrostatic force, so as not to transfer the fogging toner. In this case, even if the fogging toner is transferred onto the print sheet, it cannot be recognized. However, in use of intermediate transfer body coated with silicone rubber as above, almost all the fogging toner is also transferred and fixed onto the print sheet, thus the image quality is degraded.
Further, as the silicone rubber has good leveling characteristic, the intermediate transfer body, coated with the silicone rubber, has a very smooth surface. Accordingly, the intermediate transfer body has a high friction coefficient with respect to the very smooth surface of the photoreceptor or the like. In the conventional color-image copying and printing, three or more color toner images are overlapped with each other to form a color image. In this case, the registration among the respective colors, i.e., positional shift significantly influences the image quality. If the surface of the intermediate transfer body is smooth, its friction coefficient with respect to the photoreceptor is high, and a slip occurs between a driving roll to drive the intermediate transfer body and the rear surface of the intermediate transfer body. Further, in a case where the driving force is increased by increasing the friction coefficient between the driving roll and the rear surface of the intermediate transfer body, if the direction of the force received from the driving roll and the direction of the force received from the photoreceptor coincide with each other, the intermediate transfer body can maintain the surface flat; however, generally, these directions do not strictly coincide due to a problem of mechanical precision, and ripples or the like are produced in the surface of the intermediate transfer body due to tension between these directions, and the intermediate transfer body cannot maintain the surface flat. As a result, a toner image on the photoreceptor surface cannot be transferred with high fidelity, and a defective image is formed.
Generally, to reduce the friction coefficient of rubber, the surface of the rubber is roughened. One of the roughening methods is performing spray coating while changing the conditions such as temperature, moisture and the distance to a spray gun, for preventing the silicone rubber from becoming mist, and viscosity or the like of the silicone rubber. Although this method produces small waves in the surface, the surface itself is still smooth. Accordingly, it is difficult to greatly reduce the friction coefficient.
Another roughening method using blast processing is blow-applying sand or steel particles so as to roughen the entire rubber surface by forming cavities in the rubber surface. However, as the friction coefficient is only slightly reduced, the transfer efficiency of fogging toner is not reduced. Further, the glossiness of image is greatly reduced, and unevenness occurs due to nonuniform roughness, accordingly, the image quality is greatly degraded.